1. Field of the Invention
This invention relates to thermosetting resin compositions useful for mounting semiconductor devices onto a circuit board, such as chip size or chip scale packages (xe2x80x9cCSPsxe2x80x9d), ball grid arrays (xe2x80x9cBGAsxe2x80x9d), land grid arays (xe2x80x9cLGAsxe2x80x9d), and the like, each of which having a semiconductor chip, such as large scale integration (xe2x80x9cLSIxe2x80x9d), on a carrier substrate. The compositions of this invention are reworkable when subjected to appropriate conditions.
2. Brief Description of Related Technology
In recent years, the popularity of small-sized electronic appliances, such as camera-integrated video tape recorders (xe2x80x9cVTRsxe2x80x9d) and portable telephone sets, has made size reduction of LSI devices desirable. As a result, CSPs, BGAs, and LGAs are being used to reduce the size of packages substantially to that of bare chips. Such CSPs, BGAs, and LGAs improve the characteristics of the electronic device while retaining many of their operating features, thus serving to protect semiconductor bare chips, such as LSIs, and facilitate testing thereof.
Ordinarily, the CSP/BGA/LGA assembly is connected to electrical conductors on a circuit board by use of a solder connection or the like. However, when the resulting CSP/BGA/LGA circuit board structure is exposed to thermal cycling, the reliability of the solder connection between the circuit board and the CSP/BGA/LGA often becomes suspect. Recently, after a CSP/BGA/LGA assembly is mounted on a circuit board, the space between the CSP/BGA/LGA assembly and the circuit board is often now filled with a sealing resin (often referred to as underfill sealing) in order to relieve stresses caused by thermal cycling, thereby improving heat shock properties and enhancing the reliability of the structure.
However, since thermosetting resins are typically used as the underfill sealing material, in the event of a failure after the CSP/BGA/LGA assembly is mounted on the circuit board, it is very difficult to replace the CSP/BGA/LGA assembly without destroying or scrapping the structure in its entirety.
To that end, techniques for mounting a bare chip on a circuit board are accepted as substantially similar to the mounting of a CSP/BGA/LGA assembly onto a circuit board. One such technique, disclosed in Japanese Laid-Open Patent Publication No. 102343/93, involves a mounting process where a bare chip is fixed and connected to a circuit board by use of a photocurable adhesive, where, in the event of failure, this bare chip is removed therefrom. However, this technique is limited to those instances where the circuit board includes a transparent substrate (e.g., glass) which permits exposure to light from the back side, and the resulting structure exhibits poor heat shock properties.
Japanese Laid-Open Patent Publication No. 69280/94 discloses a process where a bare chip is fixed and connected to a substrate by use of a resin capable of hardening at a predetermined temperature. In the event of failure, this bare chip is removed from the substrate by softening the resin at a temperature higher than the predetermined temperature. However, no specific resin is disclosed, and there is no disclosure about treating the resin which remains on the substrate. Thus, the disclosed process is at best incomplete.
As pointed out in Japanese Laid-Open Patent Publication No. 77264/94, it is conventional to use a solvent to remove residual resin from a circuit board. However, swelling the resin with a solvent is a time consuming process and the corrosive organic acid ordinarily used as the solvent may reduce the reliability of the circuit board. Instead, that disclosure speaks to a method for removing residual resin by irradiation with electromagnetic radiation.
Japanese Laid-Open Patent Publication No. 251516/93 also discloses a mounting process using bisphenol A type epoxy resin (CV5183 or CV5183S; manufactured by Matsushita Electric Industrial Co., Ltd.). However, the removal process so disclosed does not consistently permit easy removal of the chip, the curing step is lengthy at elevated temperatures, and the process generally results in poor productivity.
Of course, mechanical methods of removing/replacing semiconductor chips from/on a substrate are known, such as by cutting the chip to be removed/replaced. See U.S. Pat. No. 5,355,580 (Tsukada).
Thermoplastic underfill resins are known for use in semiconductor chip attachment. See U.S. Pat. No. 5,783,867 (Belke, Jr.). However, such thermoplastic resins tend to leak under relatively modest temperature conditions. In contrast, thermosetting resins cure into a matrix which ordinarily have greater thermal stability under end use operating temperatures.
U.S. Pat. Nos. 5,512,613 (Afzali-Ardakani), 5,560,934 (Afzali-Ardakani), and 5,932,682 (Buchwalter), each refer to a reworkable thermoset composition based on a diepoxide component in which the organic linking moiety connecting the two epoxy groups of the diepoxide includes an acid cleavable acyclic acetal group. With such acid cleavable acyclic acetal groups forming the bases of the reworkable composition, a cured thermoset need only be introduced to an acidic environment in order to achieve softening and a loss of much of its adhesiveness.
U.S. Pat. No. 5,872,158 (Kuczynski) refers to thermosetting compositions capable of curing upon exposure to actinic radiation, which are based on acetal diacrylates, and reaction products of which are reported to be soluble in dilute acid.
U.S. Pat. No. 5,760,337 (Iyer) refers to thermally reworkable crosslinked resins to fill the gap created between a semiconductor device and a substrate to which it is attached. These resins are produced by reacting a dienophile (with a functionality greater than 1) with a 2.5-dialkyl substituted furan-containing polymer.
International Patent Publication No. PCT/US98/00858 refers to a thermosetting resin composition capable of sealing underfilling between a semiconductor device including a semiconductor chip mounted on a carrier substrate and a circuit board to which said semiconductor device is electrically connected. The composition includes about 100 parts by weight of an epoxy resin, about 3 to about 60 parts by weight of a curing agent, and about 1 to about 90 parts by weight of a plasticizer. Here, the area around the cured thermoset is to be heated at a temperature of about 190 to about 260xc2x0 C. for a period of time ranging from about 10 seconds to about 1 minute in order to achieve softening and a loss of much of its adhesiveness.
U.S. Pat. Nos. 5,948,922 (Ober) and 5,973,033 (Ober) each refer to a certain class of compounds having tertiary oxycarbonyl linkages, and compositions based on such compounds, which when cured provide decomposable compositions capable of being reworked.
Notwithstanding the state of the art, it would be desirable for an underfilling sealing material to provide good productivity and thermal shock properties, while allowing the substrates with which it is to be used to be readily processed and easily separated from a semiconductor device without application of acidic media or elevated temperature conditions that may compromise the integrity of the semiconductor devices remaining on the substrate or the substrate itself.
The present invention provides a curable composition, which is reworkable through thermal decomposition under exposure to temperature conditions in excess of those used to cure the composition, with the curable composition including a compound having at least one thermally cleavable linkage and which is selected from: 
where each R is independently selected from hydrogen, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, C1-4 alkoxy, halogen, cyano and nitro; each R3 is independently selected from hydrogen, methyl, ethyl, propyl, and isopropyl; R1 and R2 are each independently selected from hydrogen, methyl, ethyl, propyl, phenyl, tolyl, and benzyl, provided that at least one of R1 and R2 is phenyl, tolyl or benzyl, and m is 1; 
where R5 is phenylene, R6 and R7 are each independently selected from methylene, ethylene, propylene, or phenylene; R8 and R9 are each independently selected from hydrogen, methyl, ethyl, and propyl, provided that both R8 and R9 cannot be hydrogen; and R10 and R11 are each independently selected from hydrogen, methyl, ethyl, and propyl, provided that both R10 and R11 cannot be hydrogen; and 
where R13 is phenylene, and R14 and R15 are independently selected from secondary or tertiary aliphatic moieties.
The invention further relates to the curable compounds themselves. Particularly desirable curable compounds within the above fomulas are given in the section entitled xe2x80x9cDetailed Description of the Invention,xe2x80x9d which follows hereinafter.
In a further aspect, the curable compound is incorporated into a thermosetting resin composition, which is used as an underfill sealant between a semiconductor device and a circuit board to which the semiconductor device is electrically connected. Such a thermosetting resin composition includes, broadly, a curable resin component, a portion of which is a curable compound having at least one thermally cleavable linkage as described above; an optional inorganic filler component; and a curing agent component including an anhydride component, a nitrogen-containing component, such as an amine compound, an amide compound, and/or an imidazole compound, and combinations thereof.
Reaction products of these compositions are capable of softening under exposure to elevated temperature conditions, such as in excess of the temperatures used to cure the composition. Such temperature exposure combined with the curable compound having at least one thermally cleavable linkage provides the reworkable aspect of this invention. The remaining components, discussed below, provide the physical properties and characteristics for the compositions and reaction products to render the compositions attractive for commercial use, particularly in the microelectronics industry. Of course, the compositions may also be used for other microelectronic assembly applications, such as the direct chip attach applications, including glob top, and dam and fill. In addition, the compositions may be used in far-flung applications, where the thermosetting epoxies, or for that matter other thermosetting or thermoplastic adhesive, coating, and sealant compositions, may be used.
For instance, the compositions may be used in the assembly of products, whose component parts have value as do the intermediate/finished products, to facilitate assembly and dissassembly thereof where defective component parts are found. In that event, the defective component part(s) may be readily removed from the intermediate/finished product(s) and be replaced without having to scrap the entire intermediate/finished product(s).
In addition, the speed with which the disassembly may proceed allows throughput to remain high. A non-microelectronic example of such a part is the assembly of prosthetic devices.
In a further aspect of the invention, the curable compound with at least one thermally cleavable linkage includes an aromatic moiety within the structure. Here, the thermally cleavable linkage of the desirable compound is desirably chosen from aromatic ester linkages and aliphatic ester linkages including an aromatic moiety within the structure. Particularly desirable are those compounds identified in formula I where at least one of R1 and R2 is phenyl, tolyl or benzyl and m is 1, as well as those compounds within formulas III, VIII, and IX given in the xe2x80x9cDetailed Description of the Invention.xe2x80x9d
The inventive thermosetting resin composition is useful as an underfilling sealing resin, and enables a semiconductor device, such as a CSP/BGA/LGA assembly which includes a semiconductor chip mounted on a carrier substrate, to be securely connected to a circuit board by short-time heat curing and with good productivity. Reaction products of the inventive compositions demonstrate excellent heat shock properties (or thermal cycle properties), and permit the semiconductor device to be easily removed from the circuit board by localized heating in the event of semiconductor device or connection failure. This makes it possible to reuse the circuit board (with the remaining functioning semiconductor devices still electrically attached) and thereby achieve an improvement in the yield of the production process and a reduction in production cost.
The compositions of this invention may also be used for microelectronic applications beyond sealing underfill, such as with glob top, die attachment, and other applications for thermosetting compositions in which rapid cure time and an extended useful working life are desirable.
Other benefits and advantages of the present invention will become more readily apparent after a reading of the xe2x80x9cDetailed Description of the Inventionxe2x80x9d section together with the figures.